- Open Access
Coupled solutions for a bivariate weakly nonexpansive operator by iterations
© Berinde et al.; licensee Springer. 2014
- Received: 12 April 2014
- Accepted: 30 June 2014
- Published: 22 July 2014
We prove weak and strong convergence theorems for a double Krasnoselskij-type iterative method to approximate coupled solutions of a bivariate nonexpansive operator , where C is a nonempty closed and convex subset of a Hilbert space. The new convergence theorems generalize, extend, improve, and complement very important old and recent results in coupled fixed point theory. Some appropriate examples to illustrate our new results and their generalization are also given.
- Convex Subset
- Nonexpansive Mapping
- Strong Convergence Theorem
- Double Sequence
- Couple Fixed Point
The study of coupled fixed points has been considered in 2006 by Bhaskar and Lakshmikantham  (see also ). A rich literature on the existence of coupled fixed points of mixed monotone, monotone and non-monotone mappings, has been developed ever since the publication of that paper (see [3–30]).
In particular, the authors establish three kinds of coupled fixed point results: (1) existence theorems (Theorems 2.1 and 2.2); (2) an existence and uniqueness theorem (Theorem 2.4); and (3) theorems that ensure the equality of the coupled fixed point components (Theorems 2.5 and 2.6).
Theorem 1 (, Theorem 2.1 and Theorem 2.6)
Let be a partially ordered set and suppose there is a metric d on X such that is a complete metric space. Let be a continuous mapping having the mixed monotone property on X.
Suppose, additionally, that are comparable. Then for the coupled fixed point , we have .
Contraction-type conditions arise naturally in connection with Lipshitzian properties of mappings in the study of nonlinear functional differential and integral equations. Therefore, coupled fixed point results for contractions have important applications in nonlinear analysis and have been applied successfully for solving various classes of nonlinear functional equations: integral equations and systems of integral equations [5, 7, 13, 17, 18, 26, 27, 31]; (periodic) two point boundary value problems [1, 10, 15, 28]; nonlinear Hammerstein integral equations ; nonlinear elliptic problems and delayed hematopoesis models ; systems of differential and integral equations ; nonlinear matrix and nonlinear quadratic equations [4, 13], initial value problems for ODE [6, 24], etc.
We note that in all the above mentioned cases, the main conclusion is drawn using (, Theorem 2.6), which guarantees existence as well as equality of components of the coupled fixed point.
On the other hand, in almost all the papers dealing with study of coupled fixed points, no attention is paid to the constructive features of such a result, i.e., there is neither explicit mention of the method by which one could approximate that coupled fixed point, nor on the order of convergence and/or error estimates of the iteration processes involved.
Moreover, there exist (mixed) monotone mappings (see Examples 1 and 3 below), which possess coupled fixed points, for which no coupled fixed point theorem existing in literature can be applied. This is mainly because all those theorems (we refer here only to the ones given in [1, 4–7, 10, 13, 15–18, 23–31]) are based on a strict contractive-type condition (1.1).
All the above observations motivate for constructive study of coupled fixed points of a bivariate mapping satisfying a weaker contractive condition of nonexpansiveness type and providing a constructive method to approximate these coupled fixed points, which we generally meet in applications, i.e., when we have equality of the coupled fixed point components.
The only paper that considers asymptotically nonexpansive bivariate mappings and the existence of their coupled fixed points is due to Olaoluwa et al. . No other attempt has been made to tackle this important problem. We find here coupled solutions for a bivariate weakly nonexpansive operator on Hilbert spaces through an iterative method. Since nonexpansive bivariate mappings are particular sub-classes of the weakly nonexpansive mappings considered in the present paper, our results also generalize, improve and complement the corresponding results obtained in .
In order to illustrate the broader scope and novelty of our results, we present appropriate examples to delineate them from the existing coupled fixed point theorems in literature and indicate their potential use in applications.
In this paper, we define the concept of nonexpansiveness for bivariate mappings as follows.
for all , where and .
A similar but stronger concept has been introduced in .
Definition 2 ()
for all .
Note that our condition (2.1) is more general than (2.2): any nonexpansive mapping F is weakly nonexpansive but the converse is not true, in general, as shown below.
Then F satisfies condition (2.1) but does not satisfy condition (2.2). Moreover, F possesses a unique coupled fixed point of the form , i.e., , but no coupled fixed point theorem established in [1, 4–7, 10, 13, 15–18, 23–31] (and in other related papers) can be applied to this function F.
First, let us note that (2.1) holds with the constants and . Suppose F satisfies (2.2).
Then, taking , in (2.2), we get , a contradiction. This proves that, indeed, F does not satisfy (2.2).
where with .
It is important to note that Opoitsev  was the first who studied coupled fixed points of bivariate mappings (see also [32, 33]) where a double Picard-type iteration sequence of the form (2.4) was used.
In order to state our main results, we need some concepts and results, adapted from the case of mono-variate operators to the case of bivariate operators.
The concept of demicompact operator has been introduced by Petryshyn  (see also  and ) for a mapping , where C is a subset of a Hilbert space H. For the bivariate case it is adapted as follows.
Definition 3 A mapping is called demicompact if it has the property that whenever and are bounded sequences in C with the property that and converge strongly to 0, then there exists a subsequence of such that and strongly.
We need the following version of the well-known Browder-Gohde-Kirk fixed point theorem (see, for example, Theorem 3.1 in ), stated here in the Hilbert space setting.
Theorem 2 Let C be a bounded, closed and convex subset of a Hilbert space H and let be a (weakly) nonexpansive operator. Then F has at least one coupled fixed point in C.
Proof Let be given by , . By the (weakly) nonexpansiveness property of F, we obtain the nonexpansiveness of T and hence, by the Browder-Gohde-Kirk fixed point theorem, it follows that . □
Remark 1 Theorem 2 shows that F has at least one (coupled) fixed point of the form , but, in general, for a bivariate mapping F it is also possible to have coupled fixed points with unequal components, i.e., such that , as shown by the following example.
Then F is weakly Lipschitzian with constants and (in the sense of Definition 1) and F possesses two coupled fixed points , with equal components and two coupled fixed points with unequal components, and .
The main result of this paper is the following strong convergence theorem for a double Krasnoselskij-type algorithm associated with bivariate weakly nonexpansive operators on Hilbert spaces.
where , converges (strongly) to a coupled fixed point of F.
Proof By Theorem 2, F has at least one coupled fixed point with equal components, .
We first show that the sequence converges strongly to zero.
By (3.7), , which shows that is a coupled fixed point of F.
Using now the inequality (3.6), with , we deduce that the sequence of nonnegative real numbers is nonincreasing, hence convergent.
Since its subsequence converges to 0, it follows that the sequence itself converges to 0, that is, the sequence converges strongly to , as . □
Remark 2 Any nonexpansive bivariate mapping is weakly nonexpansive. Hence, by Theorem 3, we obtain Corollary 2.3 in .
We now introduce the concept of demicompactness at a point for a bivariate operator (adapted from the original definition of Petryshyn ).
Definition 4 A map F of into H is said to be demicompact at if, for any bounded sequence in C such that as , there exist a subsequence and an x in C such that as and .
Remark 3 Clearly, if F is demicompact on C, then it is demicompact at 0 but the converse is not true.
The demicompactness of F on the whole C in Theorem 3 may be weakened to the demicompactness at 0, provided that F is continuous.
Theorem 4 Let H be a Hilbert space, C a closed, bounded and convex subset of H, and a weakly nonexpansive mapping such that F is demicompact at 0.
Then the Krasnoselskij-type double sequence given by in C and (3.1) converges (strongly) to a coupled fixed point of F.
Proof Note that in the proof of Theorem 3, we actually used the demicompactness of F at 0, so the arguments used there can be applied here. □
Remark 4 The conclusion of Theorem 4 remains true if instead of the demicompactness of F at 0, we suppose maps closed sets in C into closed sets of H (see ).
If in Theorems 3 and 4, we remove the demicompactness assumption, then (see ), the Krasnoselskij iteration does no longer converge strongly, in general, but it could converge (at least) weakly to a fixed point, as shown in the next theorem, which extends Theorem 3.3 in .
Denote by , the set of all coupled fixed points of F with equal components, i.e., .
converges weakly to p, for any .
Remark 5 The assumption in Theorem 5 may be removed to obtain the following more general result (similar to Theorem 3.4 in ).
converges weakly to a coupled fixed point of F.
is closed, convex, and, hence, weakly compact.
giving a contradiction.
which means that . □
Since , it follows that converges to as , for any initial value .
This shows that, for weakly nonexpansive mappings, by using a Krasnoselskij-type iteration we can reach the convergence, while, by means of Picard-type iterations, this cannot be obtained, in general. Indeed, in this case, the Picard-type iteration associated with F is given by , , which is not convergent (except for the case ).
Remark 6 It is important at this stage to say that the coupled fixed point theorems existing in literature, see [1, 4–7, 10, 13, 15–18, 23–31] (only a short list is cited here), cannot be applied to the bivariate functions in Examples 1 and 3.
and it is easily seen that still converges to , the unique coupled fixed point of F, for all .
This also indicates that it is not necessary to consider only the case of a double sequence with equal components in Theorems 3-6 (but the proof of a convergence theorem for such an iterative method will be essentially different from the one given in this paper).
To conclude this paper, we note that, for the general case of a weakly nonexpansive bivariate mapping F, the Picard-type iteration process (2.4) does not generally converge or, even if it converges, its limit is not a coupled fixed point of F, but the Krasnoselskij-type iteration process always converges to a coupled fixed point of F.
In the same way, we can prove convergence theorems for iterative methods of Krasnoselskij type for tripled fixed points, quadruple fixed points etc. of weakly nonexpansive mappings (see [8, 9, 11, 14, 19–21, 38–51], and references therein).
The paper has been finalized during the first author’s visit of Department of Mathematics and Statistics, King Fahd University of Petroleum and Minerals. He gratefully thanks the host for kind hospitality and excellent work facilities offered. The first and third authors’ research was supported by the Grants PN-II-RU-TE-2011-3-239 and PN-II-ID-PCE-2011-3-0087 of the Romanian Ministry of Education and Research. The second author is grateful to KACST, Riyad, for supporting research project ARP-32-34. We are grateful to the anonymous referee for the extremely careful reading of the first version of this manuscript and for the corresponding suggestions for improvement, and especially for pointing out a few (non-fatal) errors in the proofs.
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